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Texas Instruments Introduction to Ultrasound Application notes
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Introduction to Ultrasound
Sahana Krishnan
Medical and industrial ultrasound systems use focal
imaging techniques to achieve imaging performance
far beyond a single-channel approach. Ultrasound
images are created by sending high voltage pulses
into human tissue. The sound generated by these
pulses echoes off of the tissues at varying amplitudes
depending on factors such as depth within the body
and type of tissue. Ultrasound technology is
manufactured to measure the voltage magnitude of
these echoes as they are collected at the receiver.
These voltages are ultimately recorded and displayed
in an image that tells what kinds of surfaces the pulses
are passing through.
Ultrasound technology does not involve as much
ionizing radiation exposure as other imaging methods
such as X-ray. However, the acoustic waves used to
generate the pulses are low energy, and there are
some difficulties when the waves need to penetrate
through thick layers of human tissue. The waves also
need amplification depending on their location within
the body. Texas Instruments has analog products to
facilitate advanced ultrasound system designs with low
power consumption, high performance and small size,
yielding portability with high-quality images. By using
an array of receivers, TI’s latest products for
ultrasound enable high definition images through time
shifting, scaling and intelligently summing echo
energy. This makes it possible to focus on a single
point in the scan region.
Texas Instruments’ AFE58xx family contains highlyintegrated analog front-end (AFE) solutions specifically
designed for ultrasound systems. The devices
integrate a complete time-gain-control (TGC) path and
a continuous wave Doppler (CWD) path to assess how
close the body structures are to the probe and to
increase the signal intensity accordingly. Various
power and noise combinations can be selected to
optimize system performance. TI’s leading devices for
these applications include the AFE5816, AFE5818,
AFE5828, and AFE5832. Each of these devices is an
analog front-end, integrating a low-power passive
mixer to create the on-chip CWD beamformer within
the machine. The AFE5816, AFE5818, and AFE5828
are all 16-channel devices while the AFE5832 is a 32channel device. The AFE58JDxx devices come with
the added features of an optional digital demodulator
and JESD204B data packing blocks following the
ADC.
When initiating a scan using an ultrasound machine, a
pulse is generated and transmitted from each of the
eight to 512 transducer elements. These pulses are
timed and scaled to illuminate a specific region of the
body. After transmitting, the transducer element
immediately switches into receive mode. The pulse,
now in the form of mechanical energy, propagates
through the body as high frequency sound waves,
typically in the range of 1 to 15MHz. At focal points
close to the surface (the near field), the receive
echoes are strong, requiring little if any amplification.
At focal points deep in the body (the far field), the
receive echoes will be extremely weak and must be
amplified by a factor of 100 or more. As the signal
travels, portions of the wavefront energy are reflected
back to the transducer/receiver.
The two largest contributors of receive noise in the
configuration are the transducer/cable assembly and
the receive low-noise amplifier (LNA). In the low-gain
mode (near field), the performance limit is defined by
the magnitude of the input signal. Within the receive
chains in TI’s AFEs, the LNA is integrated with a
voltage-controlled attenuator (VCA) and a
programmable gain amplifier (PGA). Low-pass filtering
is typically used between the VCA/PGA and ADC as
an anti-aliasing filter and to limit the noise bandwidth.
For analog-to-digital converters (ADCs), channel
integration and SNR are two of the most important
issues. Each channel of the ADC_CONV die in the
AFE58xx series has a high-performance ADC with
programmable resolution of 10-, 12- or 14- bits. The
ADC provides excellent signal-to-noise ratio (SNR) at
low-channel gain by achieving an SNR of 75-dBFS in
14-bit mode, and 72-dBFS in 12-bit mode. The output
interface of the ADC is also a low-voltage differential
signaling (LVDS) or JESD204B interface that can
easily connect with low-cost field-programmable gate
arrays (FPGAs).
The digital front-end part of the system takes in data
from a number of ADCs. The channel count can vary
from eight for ultra-portable systems to 512 for highend devices. The main function of the digital front-end
is to perform focusing at a given depth and direction.
This beamforming is performed by resampling the
ADC output at a higher rate, properly delaying the
resampled data, multiplying by a weight (apodization
factor), and then summing all the weighted and
delayed outputs.
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Introduction to Ultrasound Sahana Krishnan
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The beamformed data is then passed through a midvarious ways. For 2D mode imaging, the envelope
processing block where various filtering is performed
data is compressed to bring it to the dynamic range of
to reduce noise and properly extract the ultrasound RF
the human eye. The data is then scan converted to the
data. This is followed by demodulation to create
final output display form and size. For doppler
complex baseband data. Adaptive processing based
processing, velocity and turbulence are estimated in
on the depth and angle of measurements can be used
the color flow mode, and power is estimated in the
to get an optimized ultrasound image. The output from
power doppler mode. All of these estimates are scan
the mid-processing stage is handled in the back-end in
converted to the final output display form and size.
The following system block diagram shows module interaction in the ultrasound end equipment.
Integrated HV Pulser
OR
Standard Signal Chain
High Voltage Linear Amplifier
HV MUX/
DEMUX
Front End
Passive LPF
Signal Chain
Power
T/R Switches
Amplifier + Filter
VCA
Low Pass
Filter
PGA
Beamformer
Control Unit
Receive
Beamformer
REF
Amplifier Stage
LNA
Transducer
Temp
Sense
Transmit
Beamformer
DAC
Core and I/O
Power
Analog Front
End
Mid
Clocking
ADC
Preprocessing
CW (analog)
beamformer
Back End
Time Gain
Control
DAC
B Mode
Processing
Spectral
Doppler
Processing
ADC
Color/Power
Doppler
Processing
Scan Conversion
Post Processing
x
x
xx
LPRF
Wi-Fi
System
Power
AC/DC Supply With Green
Mode Controller
Power
Supply Voltage
Supervisor
Main Power Supply
Ultrasound OS/UI
RS232
USB
1394
x
IDE/
ATA
MMC/
SDIO
x
Audio Output
LEGEND
x
Processor
Display
Display
Common Interfaces
x
x
Audio
Amp
Backlight
Touchscreen
Power
Interface
ADC/DAC
Wireless Connectivity
Clocks
Amplifier
Other
The following image shows TI’s AFE5818 signal chain and how it connects to the rest of the ultrasound topology
(denoted by the red arrows).
T/R Switches
SPI IN
Receive
Beamformer
AFE5828 Device
(1 of 16 Channels)
SPI Logic
SPI OUT
LVDS
LNA
16X CLK
1X CLK
16
Phases
Generator
VCAT
CW Mixer
16 x 8
Crosspoint
SW
3rd Order LP
Filter
PGA
Summing
Amplifier/ Filter
Reference
CW I/Q
Output
Differential
TGC VCNTL
Reference
1X CLK
Time Gain
Control
2
12- to 14bit ADC
DAC
Introduction to Ultrasound Sahana Krishnan
SLOA252 – November 2017
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